1. Field of the Invention
The present invention relates to a charging method. More particularly, the present invention relates to a method for charging a battery module.
2. Description of Related Art
Along with increasing improvement of performance of processors, application programs, and drawing function, the electric power required by portable computers is gradually increased. In order to make the batteries reach the electric power requirement during the operation of the portable computers, usually the battery is designed to assemble a plurality of parallel-connected battery core sets in a battery module through serial connection, thereby gathering enough electric power for the portable computers.
With the increment of battery capacity, how to quickly and safely charge batteries has become an important issue for the manufacturers. Due to the special design of the battery module, the electric power varies at different time points or in different charging environments, so many corresponding charging methods are developed directed to the characteristics of the battery module.
In ROC patent No. 250,713, a power source management circuit is provided for controlling charging parameters supplied to a battery. FIG. 1 is a block diagram of a conventional power source management circuit. As shown in the figure, the power management circuit 100 includes a power control circuit 110, a control signal generating circuit 120, and a current control circuit 130. The power control circuit 110 is used to supply a power control signal representing the output power level of the DC power source, and the control signal generating circuit 120 reduces the charging parameters supplied to the battery when the power output level exceeds a predetermined power threshold level. In addition, the current control circuit 130 supplies a current control signal representing the output level of the DC power source current. The control signal generating circuit 120 further compares the current control signal with the current threshold signal representing the current threshold level. When the current output level exceeds the current threshold level, the control signal generating circuit 120 further reduces the charging parameters supplied to the battery. It is known from the above that in the conventional art, when the electric power variation during the charging of the battery reaches a current threshold level, the electric power supplied to the battery charging is decreased.
FIG. 2 is a schematic view of a conventional battery charging state. Referring to FIG. 2, the charging method can be divided into two stages. In the first stage (t=0−t1), a constant current charging is adopted, and a charging curve 210 represents the variation of voltage VPC of the battery module. When the voltage VPC of the battery module reaches a voltage Vinc supplied by the charger, a second stage is entered (t=t1−t2), and a constant voltage charging is used instead until the battery module is full (t=t2). In the method, the overall voltage of the battery module is charged, and the charging voltage supplied to each of the parallel-connected battery core sets cannot be adjusted according to the charging state thereof, such that the battery module may still be charged even when the voltage of the parallel-connected battery core set exceeds a safety value. Thus, not only the life of the parallel-connected battery core set is reduced, but also the battery is in danger of being overcharged.
FIG. 3 is a schematic view of a conventional battery charging state. Referring to FIG. 3, the difference from the previous method is that in the charging method, the voltage of each parallel-connected battery core set in the battery module is respectively measured, and the charging type of the overall battery module is adjusted according to the maximum value of the measured voltage. More particularly, in the first stage (t=0−t1) of this method, a constant current charging is also adopted, in which a curve 310 represents the variation of the maximum voltage Vemax of each parallel-connected battery core set in the battery module, and a curve 320 represents the variation of the minimum voltage Vemin of each parallel-connected battery core set in the battery module. When the voltage Vemax of the parallel-connected battery core set reaches a nominal voltage Vcoff endured by the parallel-connected battery core set, the electric power supplied by the charger is turned off. Here, the voltage Vemax of the parallel-connected battery core set starts to decrease. Until the voltage Vemax of the parallel-connected battery core set decreases to a voltage lower limit Vcon of the parallel-connected battery core set, the electric power supplied by the charger is turned on to increase the voltage Vemax of the parallel-connected battery core set. When the voltage Vemax of the parallel-connected battery core set reaches a nominal voltage Vcoff, the power supply is turned off. The charger is opened and closed repeatedly until all the parallel-connected battery core sets of the battery module are full. In a second stage (t=t1−t2), the magnitude of the charging current is determined according to the variation of the minimum voltage Vemin of each parallel-connected battery core set in the battery module. When the minimum voltage Vemin of the parallel-connected battery core set exceeds a voltage Vincc supplied from the charger to the parallel-connected battery core set, the second stage is entered, and the magnitude of the charging current is reduced gradually. Similarly, with the variation of the voltage Vemax of the parallel-connected battery core set, the power is discontinuously supplied until the battery is full (t=t2). By adopting the above method, although a single parallel-connected battery core set can be prevented from being overcharged, it takes a long time to fulfill the electric power of the battery module with the discontinuous supply of the power source, and the frequent charging and discharging of the battery may shorten the service life of the battery, which is not the optimal charging method.